Polymeric acrylonitrile films and method of making same

ABSTRACT

Method of preparing films of acrylonitrile polymer exhibiting very high tensile strength, stiffness, optical clarity, and outstanding gas barrier properties. Method entails extruding a hot, concentrated solution of said polymer in a solvent therefor that is water-soluble, e.g., dimethyl sulfoxide, onto a film-supporting surface, e.g., a flat surface, or a drum, which, preferably, has a low energy surface such as provided by wetting the surface with aqueous dimethyl sulfoxide, or coated with polytetrafluoroethylene, or other suitable means to thereby form a film. The resulting film is then contacted with an aqueous medium, e.g., water to remove the solvent (e.g., dimethyl sulfoxide) and partially replace it with water, this treatment being effected under conditions that will prevent the film from curling or puckering. The resulting film is composed essentially of acrylonitrile polymer and water, and is ignition resistant. This film can be then biaxially stretched in a substantially non-drying environment at 100° C. or less, preferably at 50°-80° C. followed by drying to remove the water, while maintaining the film under constrained conditions to prevent or limit the degree of lateral and longitudinal shrinkage. The final film thus produced has a very high tensile strength and stiffness and very low gas permeability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of Ser. No. 676,843 filed Apr. 14, 1976now U.S. Pat. No. 4,066,731 entitled Method of Making PolymericAcrylonitrile Films.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to films of polymeric acrylonitrile. Moreparticularly, the invention relates to polyacrylonitrile films and to amethod of their preparation, which films are characterized by extremelyhigh tensile strength and stiffness, and by outstanding gas barrierproperties. The invention also relates to films of acrylonitrilepolymerwater, the water being present within the film in an amount of atleast 20% by weight, such films being highly fire resistant.

2. Description of the Prior Art

Although polyacrylonitrile has been known and available since the later1940's, it has not been successfully utilized commercially in film form,probably because of its "intractability", in that it cannot be meltedwithout decomposing, so that conventional hot melt extrusion techniquesare simply inapplicable.

Shaped articles of polyacrylonitrile have been described in priorpatents. See, e.g., U.S. Pat. Nos. 2,858,290 and 3,094,502, whichdescribe polymerizing acrylonitrile in dimethyl sulfoxide to provide aspinning solution of polyacrylonitrile in the foregoing solvent,followed by extrusion. See also U.S. Pat. No. 3,437,717, which describesthe preparation of a film made from a copolymer of acrylonitrile and apolyalkenyl monomer, the film being prepared by dissolving the copolymerin a polar organic solvent such as dimethyl formamide or mixtures ofdimethyl formamide with dimethyl sulfoxide, followed by casting the filmfrom the foregoing solution, removing part of the solvent, stretchingthe film biaxially, and removing most of the remaining solvent by airdrying.

SUMMARY OF THE INVENTION

I have discovered a method of preparing films of acrylonitrile polymerexhibiting very high tensile strength and stiffness, optical clarity,and outstanding gas barrier properties. This method entails applying ahot, concentrated solution of the foregoing polymer in a solventtherefor that is water-soluble (e.g., dimethyl sulfoxide) onto afilm-supporting surface, e.g., a cooled, flat surface or a cooledrotating drum, to thereby form a film. An embodiment for applying thesolution onto the film-supporting surface includes extrusion of thesolution through a slot die as well as other means suitable for castingthe solution onto a film-supporting and film-forming assembly.Desirably, the film-supporting surface is a low energy surface such asmay be provided by having said surface wetted by an aqueous solution ofdimethyl sulfoxide in water, or by having the surface coated with amaterial such as polytetrafluoroethylene, and stripping the resultingfilm from said surface.

The resulting film containing the solvent (e.g., dimethyl sulfoxide) isthen subjected to contact with water, preferably while being maintainedunder constrained conditions so as to prevent or limit lateral orlongitudinal shrinkage thereof, to thereby remove the solvent (e.g.,dimethyl sulfoxide) present. This results in a film composed essentiallyof acrylonitrile polymer and water. This film per se is highly resistantto ignition because of the large amount of water present therein, and issuitable for use as a semi-permeable membrane, or as a plate separatorfor batteries.

I have further found that when the foregoing film is subjected tobiaxial stretching under substantially non-drying environment, followedby drying the film, while under restraint so as to prevent or limitshrinkage, until substantially all of the water present has beenremoved, there results a final film that exhibits an exceptionally hightensile strength and stiffness and an exceptionally low permeability togases.

BRIEF DESCRIPTION OF THE DRAWINGS

My invention will be best understood from the following detaileddescription taken in conjunction with the accompanying drawings wherein

FIG. 1 is a schematic illustration of an apparatus that may be utilizedin the method of my invention;

FIG. 2 is an illustration in greater detail of the apparatus that isemployed in the water washing of the film of polymeric acrylonitrile;

FIGS. 3 and 4 are schematic illustrations of the washing apparatus inraised and lowered positions so as to easily accommodate the film;

FIG. 5 is a schematic illustration of a baffle arrangement for promotingthe washing in well defined stages;

FIG. 6 is a plan view taken along the line 6--6 of FIG. 5; and

FIG. 7 is a plan view taken along the line 7--7 of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Considering the monomeric material from which the polymeric film isprepared, the preferred monomer is, of course, acrylonitrile.

In the following discussion, reference will be to films made from thehomopolymer of acrylonitrile. It is, however, to be understood that myinvention contemplates, in addition, the preparation of films that aremade from copolymers of acrylonitrile with other ethylenicallyunsaturated monomers that are copolymerizable therewith, wherein saidcopolymers predominate in units derived from the acrylonitrile monomer.

Suitable ethylenically unsaturated monomers copolymerizable withacrylonitrile include, for example, alkyl acrylates such as methylacrylate, butyl acrylate, and the like. Generally, I prefer to usepolyacrylonitrile homopolymer. Where a copolymer is employed, I preferthat the units in said copolymer derived from the acrylonitrile monomerbe present in an amount of at least about 80 percent by weight, andpreferably in an amount of at least 95 percent by weight.

The polyacrylonitrile (or acrylonitrile copolymer, as the case may be)may be prepared in any conventional manner, as by emulsionpolymerization, suspension polymerization, or solution polymerization.However, inasmuch as the method of my invention utilizes a solution ofpolyacrylonitrile in a solvent that is water soluble as the precursor ofthe polymer film, it clearly is most efficient to effect thepolymerization in solution wherein the solvent is dimethyl sulfoxide.

As noted, the preferred solvent employed for the solution polymerizationis dimethyl sulfoxide. Alternatively, the solvent may be succinonitrileor a mixture of dimethyl sulfoxide and dimethyl sulfone wherein at least50 percent by weight of said mixture is dimethyl sulfoxide.

Solution polymerization of acrylonitrile in dimethyl sulfoxide is knownin the art. See, e.g., U.S. Pat. No. 3,094,502 previously referred to.

It is generally advantageous to polymerize the acrylonitrile monomerwhen dissolved in the solvent, preferably dimethyl sulfoxide, in anamount between about 30 and 60 percent by weight, based on the totalweight of the solution. The optimum concentration of dissolved monomerfor employment in specific instances may vary with such factors as themolecular weight which is desired to be obtained in the polymerizedproduct, the concomitant viscosity that is desired to be achieved in thedirectly formed extruding solution, and the extrusion conditions.

In preparing the film-forming system, the polymerization is facilitatedby use of a suitable vinyl polymerization catalyst.

By the expression "vinyl monomer polymerization catalyst" is meant thewell known free radical catalysts conventionally used in vinyl monomerpolymerization processes. In this class, there are included organic azocompounds as disclosed in U.S. Pat. No. 2,471,959 wherein the azo,--N═N--, group is acyclic and bonded from both of the nitrogens tocarbons which are aliphatic in nature and at least one of which carbonsis tertiary; diacyl or diaryl peroxides, such as benzoyl peroxide,t-butyl perbenzoate, cyclohexanone peroxide, and others. They may bethermally decomposed or a redox system may be used, such as a peroxidewith SO₂ and an oxidizable heavy metal iron, such as Fe⁺⁺, or anN,N'-dialkyl aniline. Salts of perdisulfuric acid are also useful in aredox system. The catalyst may be employed in a conventional manner andin an amount as will be apparent to those skilled in the art. Generally,from 0.1 to about 1% by weight based on the monomer is preferred.

The extruded melt desirably should be substantially free of the volatilemonomer. This can be accomplished by removing the residual monomer froma normal polymerization, as by vacuum distillation, or by conducting apost-polymerization at elevated temperatures to attain substantially100% conversion.

Ordinarily, the polymerization may be terminated with formation of asuitable film-forming solution in a time period which rarely exceeds 60minutes and often may be as short as about 40 minutes or less.

Advantageously, the film-forming systems may be prepared by polymerizingthe dissolved monomeric material with catalytic assistance underatmospheric pressure at a temperature between about 80° and 150° C. fora period of time which is between about 30 and 60 minutes. The equipmentemployed for the polymerization should be equipped with an efficientmeans for agitation.

If the polyacrylonitrile is prepared by other than solutionpolymerization in dimethyl sulfoxide, an embodiment for practice of thisinvention includes recovering the polymer from the solution, washing thepolymer, and then dissolving it in dimethyl sulfoxide.

In general, it is preferred that the percent conversion of acrylonitrilemonomer to polyacrylonitrile be at least about 90%, and preferably ofthe order of 95-98%. The polymer will generally have a molecular weightof at least about 80,000. More particularly, the inherent viscosity ofthe polymer at 0.5 g/100 ml dimethyl sulfoxide, should be from about0.85 to 1.7.

The polymer solution is then extruded through a suitable extrusionmeans, e.g., a slot die and cast onto a cool surface. In instances whereit is desired to produce a thin smooth film, it is preferred that thedrum surface be one that is not easily wet by the extrudate to avoidobjectionable striation in the product film. Embodiments to effect sameinclude continuously wetting the surface with an aqueous solution ofdimethyl sulfoxide, or using a low energy casting surface, e.g., a drumcoated with a polytetrafluoroethylene.

The concentration of the casting solution (that is to say, the solutionof polyacrylonitrile in the solvent, e.g., dimethyl sulfoxide) ispreferably at least 30 percent by weight of polyacrylonitrile, andpreferably from about 40 to 60 percent by weight.

The concentration of the aqueous solution of dimethyl sulfoxide that maybe utilized to continuously wet the flat casting surface may be fromabout 30 to 70 percent by weight of dimethyl sulfoxide.

As a result of the foregoing casting operation, one obtains apolyacrylonitrile film that contains therein substantial amounts of thesolvent (e.g., dimethyl sulfoxide). Typically, this film will contain atleast about 40 percent by weight of the solvent and, preferably, fromabout 50 to 65 percent by weight thereof.

In accordance with a further aspect of this invention I have discoveredthat when the film is contacted with an aqueous medium as for example bycountercurrent extraction with an aqueous medium to remove the solvent(e.g., dimethyl sulfoxide), there results a film product having highresistance to ignition as a result of its high water content.

As noted, it is desirable that the treatment (e.g., countercurrent watertreatment) of the film to remove or partially replace the solvent withwater be carried out under conditions wherein the film is maintainedunder constraint. Thus, the film should be prevented, during the periodof the water treatment, from curling or puckering, so that it remainssubstantially flat. Thus, non-flatness that would result from puckeringor curling would interfere with subsequent processing operations.

After the foregoing water treatment, there results a film composed ofpolyacrylonitrile and water, with the water typically being present inan amount of from about 20-75 percent by weight. It should be noted thatthis water is not free. Rather, it appears that the water has enteredthe interstices and voids present in the film on a molecular scale sothat it is an integral part of the film.

Such film is, by virtue of its high water content, highly resistant toignition. This film has a wide variety of end use applications. Forexample, such a film is highly suitable as a plate separator forbatteries, as a semi-permeable membrane for use in dialysis or reverseosmosis, as a transparent wrap for produce, etc.

In accordance with a further aspect of my invention, I have discoveredthat the foregoing film, composed essentially of polyacrylonitrile andwater, may subsequently be transformed by a series of steps into a finalfilm consisting of polyacrylonitrile, which final film exhibitsextraordinarily high tensile strength and extraordinarily low gaspermeability.

In accordance with this aspect of my invention, the foregoing film isfirst subjected to biaxial stretching. The biaxial stretching ispreferably carried out in a moist environment, e.g., in the presence ofsteam, moist gas, or in hot water. The biaxial stretching can be carriedout in a one-step operation wherein the film is simultaneously subjectedto stretching in two directions that are at right angles to one another.Alternatively, the biaxial stretching can be carried out in stepwisefashion, as first by longitudinal stretching and thereafter by lateralstretching, or vice versa. In general, the biaxial stretching should becarried out such that the amount of stretch imparted to the film in eachdirection is at least about 1.5 times the original dimension prior tostretching and, generally, from about 1.5 to about 4 times. Preferably,the biaxial stretching is carried out such that the stretch in eachdirection is at least two times the original dimension.

One means for effecting biaxial stretching simultaneously in bothdirections is to clamp the film between two plates, one plate having alarge circular hole therein and the other plate having a small hole forthe introduction of compressed air. The foregoing assembly is thenimmersed in hot water. At water temperatures of at least 40° C. up toabout 100° C., compressed air is admitted under sufficient pressure tocause the film to expand through the large hole so as to form a"bubble". Within that temperature range, it is highly preferable thatthe higher the temperature the more promptly should the air admittal beinitiated. For example, at above 70° C., the air admitted is desirablysubstantially immediate. The degree of stretch is, of course, determinedby the size of the ultimate bubble. After a bubble of desired size hasbeen formed, and while maintaining the air pressure so as to retain thebubble, the entire assembly may then be immersed in cold water so as to"fix" the bubble structure. The bubble portion may then be cut away(this being the portion that was subjected to true biaxial stretching)and then dried while under restraint, as in an oven. During the drying,the film, which initially exhibited the spherical curvature defining thebubble, will shrink and become flat as it loses its water. The resultantflat film is dimensionally stable and has very high tensile strength andextremely low gas permeability.

Rather than the foregoing "bubble" technique, other means well known tothe art may be used. Thus, the biaxially oriented films embodied hereinmay, for example, be produced by sequentially orienting the film as, forexample, stretching in a longitudinal direction (machine direction)followed by stretching in a lateral direction (transverse direction) orby subjecting the film to orientation simultaneously in both thelongitudinal and transverse direction. Specific apparatus and techniquesfor effecting biaxial orientation of the film embodied herein include,for example, the drafter-tenter frame arrangement as disclosed in U.S.Pat. No. 3,437,717 for sequentially imparting machine direction andtransverse direction orientation; and the various means disclosed in,for example, the articles by J. Jack in British Plastics, June 1961,pages 312-317 and July 1961, pages 391-394 for sequential orsimultaneous biaxial orientation of plastic films.

I have discovered that if the film which may contain, e.g., 45% water asit emerges from the washer, is preheated to a stretching temperature,e.g., 100° C., in hot water or steam and is allowed to remain at thattemperature for a substantial period of time, say two minutes, its watercontent will drop substantially. Film having a low water content, e.g.,as low as about 20%, normally provides difficulty in stretching withoutbreaking. Moreover, such a film develops a white or bluish haze. Thus,to achieve smooth stretching and a haze-free final product, the "wet"film (i.e., film composed of polyacrylonitrile and water) should berapidly heated while preventing substantial evaporation from occurring,and then stretching the film immediately, before the film can adjust toits new environment and lose much of its water. The temperature at whichthe stretching is effected should be less than 200° F. and, preferablyless than 160° F. Preferably, after the film has been heated to thedesired stretching temperature, the stretching should be commencedimmediately, e.g., less than three seconds after the desired stretchingtemperature has been attained.

The oriented film obtained after the biaxial stretching will stillcontain water therein. If this film is dried without being maintainedunder restrained conditions, it shrinks appreciably. Thus, it can beused as a shrinkable film for packaging. It can then be dried while onthe package, as by the application of heat. Alternatively, it can bepermitted to dry during storage and/or shipping. The final dry film hasexcellent gas-barrier properties.

Apart from an end use as a shrinkable film, it is preferred to carry outdrying of the oriented film while either preventing it from shrinking orelse permitting it to shrink only a controlled amount.

The particular drying technique is not critical. Thus, the followingdifferent modes of drying have all proven satisfactory: 30 seconds inhigh velocity impinging air at 350° F.; 15 seconds exposure three inchesaway from a battery of 1000 watt quartz tube radiant heaters spaced oneinch apart; or two minutes in a Fisher Convection oven at 350°-400° F.(very low air velocity).

The process of my invention will now be described in further detail,with reference to the accompanying drawings.

Referring to FIG. 1, there is shown my overall apparatus, which includesa polymerization reactor generally designated by the reference numeral10, an extruder 20, a casting drum 30, and a washer 40.

Considering first the polymerization phase, suitable monomer(acrylonitrile), solvent (dimethyl sulfoxide), and polymerizationinitiator are admixed in monomer feed tank 12. They are then pumped bypump 14 to tubular reactor 10 and into the top of the annular reactionzone 16. The reactor 10 is equipped with an agitator 18, equipped with apair of screw flight scraper blades 21.

Tubular reactor 10 is surrounded with a water jacket 22 wherein thewater can be heated to boiling, as by heaters 24 and the pressure in thejacket 22 is controlled by means of a vacuum pump.

Extruder 20 withdraws polymer from the bottom of the reactor 10, and asit does so, the reactor is maintained charged with monomer feed by alevel controller [not shown].

Screw extruder 20 is jacketed with heaters 26, and is driven by avariable speed drive 28. Accordingly, as polymer dissolved in dimethylsulfoxide enters the extruder from the reactor it is passed through theextruder, into slot extrusion die 32, and is extruded therefrom in theform of a thin polymeric film. This film extrudate is cast upon drum 30,which drum rotates in a counter clockwise direction, thereby passing thecast film 33 between the drum and nip roll 34.

In accordance with an important aspect of the invention, the castingdrum 30 is continuously wetted with an aqueous solution of dimethylsulfoxide, or alternatively, the drum is provided with a coating ofpolytetrafluoroethylene. If the aqueous dimethyl sulfoxide is used, asschematically shown in FIG. 1, tank 36 contains aqueous dimethylsulfoxide solution, which solution passes from the tank through line 38and directly to the casting drum 30.

A container 21 is disposed beneath casting drum 30. In this container isaqueous dimethyl sulfoxide, which may be continuously introduced viaconduit 23. As the drum rotates, the aqueous dimethyl sulfoxide incontainer 21 helps to coagulate the film on the drum and also serves asa first washing for the film, thereby enhancing the film's strength andrendering it more readily strippable from the drum. Conduit 25 permitsthe continuous withdrawal of aqueous dimethyl sulfoxide from container21.

As shown in each of FIGS. 1 and 2, the film 33 then passes to the washer40 via guide roll 42.

Washer 40 contains therein a multiplicity of pairs of steel-rubber niprolls for the purpose of conveying the polymer film through the washerwhile at the same time restraining the film from both longitudinal andlateral shrinkage.

As shown in FIG. 2, the washer contains four steel rolls 44a, 44b, 44c,44d, and three rubber rolls 46a, 46b, 46c disposed between the steelrolls and in contact with them. The four steel rolls are geared togetherand positively driven so as to each turn at the same speed, therebypreventing substantial longitudinal shrinkage of the polymer film. Thethree rubber rolls 46 are idler rolls. By virtue of the tight frictionalengagement between adjacent rolls, any tendency toward lateral shrinkageof the polymer film is effectively reduced.

I have found that it is advantageous to mount the rubber rolls 46 onvertical slidable supports (not shown) so that these rollers can readilybe raised or lowered. As illustrated in FIG. 3, the rubber rolls areraised so as to permit easy introduction of the polymer film 33. Thethree rubber rolls 46 are then lowered (FIG. 4) so as to "nip" thepolymer film tightly between the rubber rolls 46 and the steel rolls 44.

As illustrated in each of FIGS. 1 and 2, fresh water is introduced intowasher 40 via conduit 48. The polymer film 33, rich in dimethylsulfoxide, is shown entering washer 40 at the left hand side.Accordingly, a countercurrent washing is effected, with the water, as itprogresses from right to left becoming ever richer in dimethyl sulfoxideand correspondingly, with the polymer film, rich in dimethyl sulfoxideas it enters from the left, gradually losing the dimethyl sulfoxide andhaving it replaced with water as the film progresses from left to right.The countercurrent wash solution, rich in dimethyl sulfoxide, is removedfrom the left side of the wash apparatus 40 by means of conduit 50.

In accordance with a further aspect of my invention, I have found thatthe foregoing countercurrent washing is best achieved when it is carriedout in well defined stages. To achieve this, and as is illustrated inFIG. 5, there are provided transversely mounted squeegees or baffles 52,54. Squeegees 52a, 52b, 52c, 52d and 52e, are mounted above the rolls,and squeegees 54a, 54b, 54c, 54d, 543, 54f, 54g are mounted below therolls. These transverse squeegees effectively prevent the wash waterfrom flowing freely from one end of the tank to the other. Thus, thesqueegees divide the washer into 12 compartments, six on the bottom(below polymer film 33) and six on the top. As shown in FIG. 6, water ispermitted to flow from one bottom compartment to another bottomcompartment via ports 56 in the bottom connected by hoses (not shown).Also, water is permitted to flow from one top compartment to another topcompartment via gaps between the top squeegees and the edge. That is tosay, and as shown in FIG. 7, the top squeegees do not extend across theentire width of the washer tank, thereby permitting labyrinthine flow ofthe water.

It should be noted that inasmuch as the polymer film is somewhatnarrower than the width of the tank rolls, the water on the top sidewill come in contact with the water on the bottom side outboard of theedges of the polymeric film, so that the bottom and top wash water arenot completely distinguishable.

As previously pointed out, after the polymer has been water washed thereresults a product that is composed essentially of polyacrylonitrile andwater, with the water typically being present in an amount of from about25 to 50 percent by weight. It is strong, stiff, tough, transparent, andessentially water-white.

The foregoing film if not employed as is, may be subsequently treated soas to remove the water and obtain a final film consisting essentially ofpolyacrylonitrile. To that end, the film is first subjected to biaxialstretching typically two and a half fold in both the longitudinal andlateral direction. As noted previously, the biaxial stretching may becarried out in either hot water at, e.g., 50° to 80° C., or in a moistatmosphere, typically steam/air mixtures. After the biaxial stretchingthe film should be dried under restrained conditions, as by drying inhot air at 100° to 200° C. Drying is preferably followed by heat settingfor a few seconds at, e.g., 200° C. The dry film is essentially free ofvolatile organic materials, such as solvent and acrylonitrile monomer.

Film made by such process generally has an oxygen permeability of0.005-0.03 cc/(day) (100 in²) (atm) at 1 mil and a water vaporpermeability of about 0.1-0.3 g/(day) (100 in²). The modulus ofelasticity in tension is about 600,000-800,000 psi, and the tensilestrength is about 20,000-30,000 psi with an elongation at break of50-70%. The films do not appear to shrink at 290° F.

The following example will further illustrate my invention. Theapparatus utilized was similar to that of FIG. 1. All parts are byweight unless otherwise indicated.

EXAMPLE

The following ingredients were mixed at room temperature.

8.38 liters of acrylonitrile (6.75 Kg) containing MEHQ (methyl ether ofhydroquinone) inhibitor (35-45 parts/million parts of theacrylonitrile).

8.88 liters of dimethyl sulfoxide (9.77 Kg).

17.0 g of alpha, alpha'-azo diisobutyronitrile.

No precautions were taken to exclude oxygen.

This mixture was pumped to the top of a cylindrical reactor, i.e., a 3inch (I.D.) carbon steel tube 30 inches long equipped with a waterjacket and a close fitting screw type agitator (double flighted screw, 6inch pitch, root diameter 11/2"), at a rate of about 70 ml/min. Thejacket temperature was maintained at about 85° C., and the speed of theagitator was 22 RPM. The reactor tube was vented to the atmosphere topermit air in the tube to escape as the reactor was filled.

When the reactor was full, the agitator speed was increased to 74 RPM,and a 1" diameter, 20 inch long extruder connected to the bottom of thereactor tube was started in operation at a rotational speed of about 30RPM. The extruder barrel temperature was maintained at 120° C. Thedischarge from the extruder was continuously passed into a slot diemaintained at 110° C. The resulting extruded sheet was continuously laiddown on the surface of a water-cooled rotating casting drum maintainedat a temperature of about 10° C. The drum was rotated at such speed thatthe film of polymer solution on the surface of the drum was about 0.01inch thick. The surface of the drum was continuously kept wet with a 50%solution of DMSO in water in order to prevent generation of transversestriations in the cast sheet.

The cooled film, which was quite strong but very rubbery, wascontinuously stripped off the casting drum and conducted through awashing device consisting of a series of seven 2-inch diameter rollers(steel, rubber, steel, rubber, steel, rubber, steel, all operating incontact, and all driven at the same speed) immersed in flowing tap waterat about 10° C. The resulting sheet, which was strong, tough,transparent, and stiff, was wound up to form a roll. The final wet sheet(8.5 mils thick) after further washing, contained 55% polyacrylonitrilehaving an inherent viscosity of 1.3 (0.5 g/100 ml DMSO) and 45%volatiles consisting essentially of water and perhaps small amounts ofdimethyl sulfoxide and acrylonitrile monomer.

A piece of the above wet sheet was then clamped between two gasketedmetal plates, one of which had a five inch diameter circular hole in itand the other of which had a hole for the introduction of compressedair. This assembly, with the sheet in it, was immersed in water at 75°to 85° C. Compressed air was immediately admitted under sufficientpressure to cause the sheet to expand out through the five inch hole toform a "bubble" about 4 1/16 inches high, thereby stretching the sheetbiaxially. Then the assembly was transferred, while maintaining airpressure in the bubble, to a cold water tank to immobilize the structureof the bubble.

The thus blown sheet was removed from the assembly and a 43/4 inchdiameter embroidery hoop was used to clamp the "crown" of the hat-shapedblown sheet, where the stretching orientation was substantially biaxialand substantially balanced in both directions (about three fold stretchin each direction). After trimming away the undesired edge portions ofthe "hat", the embroidery hoop with its piece of wet oriented film wasdried in a 200° C. low air velocity oven for 120 seconds. During thisdrying operation, the film, which had possessed a spherical curvature,shrank, became flat and taut, lost its water, and became dimensionallystable.

The resultant film, and other films made by the same technique, werefrom 0.00032 to 0.00062 inches thick, were clear and water-white, andhad the following properties:

Young's modulus (Kpsi): 725-833

Ultimate Tensile Strength (Kpsi): 28-39

Elongation at Break (%): 36-55

Oxygen permeability (dry oxygen at room temp: cc/(100 in²) (day) (atm):0.006

Water vapor permeability (50% RH, 73° F.): g/(100 in²) (day): 0.11

Shrinkage at 290° F.: Negligible

Variations can, of course, be made without departing from the spirit andscope of the invention.

Having thus described the invention, what is desired to be secured byLetters Patent and hereby claimed is:
 1. A biaxially-oriented dry filmconsisting essentially of acrylonitrile homopolymer having opticalclarity, low oxygen permeability, low water vapor permeability, atensile strength of about 28,000 p.s.i. to about 39,000 p.s.i., and amodulus of elasticity of about 725,000 p.s.i. to about 833,000 p.s.i. 2.A dry film according to claim 1 which is essentially free of solvent andacrylonitrile monomer.
 3. The film of claim 1 wherein the film has anoxygen permability of from about 0.005 to about 0.03 cc/100 in.²/day/mil/atm. and wherein the film has a water vapor permeability offrom about 0.1 to about 0.3 gram/100 in.² /day/mil.
 4. A biaxiallyorientable film prepared by the steps comprising applying onto a filmsupporting cooled surface a concentrated solution of an acrylonitrilepolymer in a solvent therefor that is water soluble to provide a filmcomprising said polymer and said solvent, and contacting the resultingfilm with an aqueous medium to substantially remove and replace withwater the solvent in the film.
 5. A biaxially orientable film, asdefined in claim 4 consisting essentially of an acrylonitrile polymerand water in amount of from about 20 to 75% based on the weight of thefilm, said water being integrally bound in the molecular interstices ofsaid polymer.
 6. A film, as defined in claim 5, that contains from about40 to 60% by weight water.
 7. A film, as defined in claim 5, wherein theacrylonitrile polymer is acrylonitrile homopolymer.
 8. A film, asdefined in claim 5, wherein said acrylonitrile polymer is a copolymer ofacrylonitrile with an alkyl acrylate, said copolymer containing unitsderived from acrylonitrile in amount of at least about 80% by weight ofsaid copolymer.
 9. The film of claim 4, which is biaxially oriented anddried under restraint.
 10. A biaxially oriented film prepared by thesteps comprising applying onto a film supporting cooled surface aconcentrated solution of an acrylonitrile polymer in a solvent thereforthat is water soluble to provide a film comprising said polymer and saidsolvent, contacting the resulting film with an aqueous medium tosubstantially remove and replace with water the solvent in the film, andbiaxially orienting the film.
 11. The film of claim 10 wherein saidsolvent comprises dimethyl sulfoxide.
 12. The film of claim 10 whereinsaid aqueous medium comprises a solution of water and dimethylsulfoxide.
 13. The biaxially stretched film of claim 10 that has beendried to substantially remove its water content.
 14. A film, as definedin claim 13, characterized by having low gas permeability properties.15. A film, as defined in claim 14, that additionally has a high tensilestrength and optical clarity.
 16. The film of claim 10 wherein thebiaxially oriented film is dried under constrained condition tosubstantially remove water therefrom.
 17. The film of claim 16 whereinthe film has an oxygen permeability of from about 0.005 to about 0.03cc/100 in.² /day/mil/atm.
 18. The film of claim 16 wherein the film hasa water vapor permeability of from about 0.1 to about 0.3 gram/100 in.²/day/mil.
 19. The film of claim 16 wherein the film has a tensilestrength of from about 28,000 p.s.i. to about 39,000 p.s.i. and amodulus of elasticity of from about 725,000 p.s.i. to about 833,000p.s.i.